Use array-meaningful ampersand to change column in foreach loop

Question

I wish to loop over this code:

\begin{tabular}{l*{8}c}
    first column &
    column a &
    column b &
    column c &
    column d &
    column e &
    column f &
    column g
\end{tabular}

But the following does not compile:

\begin{tabular}{l*{8}c}
    first column
    \foreach \i in {a,b,c,d,e,f,g,h}{%
        & column \i%
    }
\end{tabular}

with an error that looks nothing like something I've written:

! Extra }, or forgotten \endgroup.
<template> \unskip \hfil }
                          \hskip \tabcolsep \endtemplate 
l.40         }

I've hope for a while that this post would help, but I'm not sure that this part of the question was finally answered there.

How do I make this loop work?

Answer 1

You can get a very handy syntax with xparse and expl3. In the second argument #1 stands as a placeholder for the various items in the list.

\documentclass[a4paper]{article}
\usepackage[margin=1.5cm]{geometry}
\usepackage{booktabs}
\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\firstrow}{mmm}
 {%#1 = first column
  %#2 = format for the other columns
  %#3 = comma separated list of values
  \cs_gset_protected:Nn \__iagolito_temp:n { & #2 }
  #1 \clist_map_function:nN { #3 } \__iagolito_temp:n
 }
\ExplSyntaxOff

\begin{document}

\begin{tabular}{l*{8}c}
\firstrow{first column}{column #1}{a,b,c,d,e,f,g,h} \\
x & 1 & 2 & 3 & 4 & 5 & 6 & 7 & 8 \\
\end{tabular}

\bigskip

\begin{tabular}{l*{8}c}
\toprule
\firstrow{first column}
  {\begin{tabular}[t]{@{}c@{}}column\\ #1\end{tabular}}
  {a,b,c,d,e,f,g,h} \\
\midrule
x & 1 & 2 & 3 & 4 & 5 & 6 & 7 & 8 \\
\bottomrule
\end{tabular}

\end{document}

Answer 2

Essentially you can't make it work using \foreach: each table cell is a group and & is like }{ which ends one group and starts a new one, at which point the internal state of the loop is lost. You either need a different loop system that doesn't require local definitions or (like the answer to the question you reference) just build up a macro with the table row then execute that macro at the end of the loop.

This shows both mechanisms, note that in the second one the loop data is managed on the parameter stack and no local (or global) definitions are made, which makes it easier to do the loop "inside" the table.

\documentclass[a4paper]{article}
\usepackage{tikz}
\addtolength\textwidth{2in}
\addtolength\oddsidemargin{-1in}
\begin{document}
\small\centering
\def\zz{first column}
    \foreach \i in {a,b,c,d,e,f,g,h}{%
      \xdef\zz{\zz & column \i}%
    }
\begin{tabular}{l*{8}c}
    \zz
\end{tabular}


\def\zzz#1{\zzza#1,\relax}
\def\zzza#1,#2\relax{&column #1\ifx\relax#2\relax\else\expandafter\zzza\fi#2\relax}

\begin{tabular}{l*{8}c}
first column \zzz{a,b,c,d,e,f,g,h}
\end{tabular}




\end{document}

Answer 3

Here is an approach using tokens and the listofitems:

Basically it makes a token list from a comma separated list and then calls the token list when you want it (using \the\tabrows):

\documentclass{article}
\usepackage{listofitems}
\makeatletter
\newtoks\tabrows
% Command for adding tokens to list
\newcommand\addrowraw[1]{\tabrows\expandafter{\the\tabrows#1}}
% Command that takes a comma separated list
% of items to be in the column of a row
% The second argument is what to do with the current column
% using the \thiscol command:
\newcommand\addrow[2]{
  % Macro to contain the new list
  \def\newrow{}
  % Read argument as list for listofitems
  \readlist\thecolumns{#1}
  % Iterate and append the ampersand and the content of each column
  \foreachitem\thiscol\in\thecolumns{
    \edef\newrow{\unexpanded\expandafter{\newrow}&#2}
  }
  % Remove first ampersand
  \edef\newrow{\expandafter\@gobble\newrow}
  % Just grouping it
  \edef\newrow{{\newrow}}
  \expandafter\addrowraw\newrow
  % Add the \cr
  \addrowraw{\\}
}
\begin{document}
  % These should be _before_ the tabular environment
  \addrow{a,b,c,d,e,f,g,h}{Col \thiscol, r1}
  \addrow{i,j,k,l,m,n,o,p}{Col \thiscol, r2}
  \begin{tabular}{l*{8}c}
     \the\tabrows
  \end{tabular}

\end{document}